The invention relates to the field of vehicle transmissions and especially to the field of vehicle transmissions with a power take off unit.
A general trend in the development of vehicle transmissions with power take off is the need for increased power output. One negative consequence of this is that the increased power generates a higher temperature in the power take off unit. Commonly a transmission comprises a main gearbox and a power take off unit. To cool the power take off unit, it is adapted with a lubrication circulation system. Usually, the lubrication circulation system is common with the main gearbox, wherein a part of the gearbox lubricant is derived into the power take off unit for cooling and lubrication. The lubricant is thereafter lead back to the main gearbox. In known designs, the lubrication circulation system can not be turned off exclusively for the power take off unit, which means that the lubrication circulation system runs for the power take off unit at all times it runs for the main gearbox.
With increased power output, more oil is needed for cooling and lubrication of the power take off unit. However, to assure the lubrication and cooling of the main gearbox, just a limited portion of the lubricant of the transmission can be used for the power take off unit. Thereby, by deriving too much of the lubricant to the power take off unit, the cooling and lubrication of the main gearbox could become deficient.
It is desirable to suggest a simple and efficient supply of lubricant to the power take off unit, without risking a deficiency in lubrication of the main gearbox.
Thus, the present invention is based on a transmission comprising a main gearbox and a power take off unit and wherein the power take off unit can be engaged with, respectively disengaged from, the main gearbox through an engagement mechanism having at least a first position engaging the main gearbox and the power take off unit and a second position disengaging the power take off unit from the main gearbox, wherein the power take off unit is provided with at least one lubricant inlet.
Further, the present invention is characterised in, that said engagement mechanism comprises a valve member, whereby when said engagement mechanism is positioned in its second position, the valve member is blocking said lubricant inlet, and thereby preventing lubricant to flow through the lubricant inlet into the power take off unit. When said engagement mechanism is positioned in its first position, i.e. is engaged with the main gearbox, the lubricant inlet is open and lubricant can flow through the lubricant inlet, into the power take off unit.
An on/off-function is thereby created for the lubricant inlet in the power take off unit. Due to the blocking of the lubricant inlet, when the power take of unit is disengaged from the main gearbox, the lubricant can be used in the main gearbox where it is needed and the lubricant is thereby used more efficiently. Further, energy is saved, since lubricant does not have to be pumped around to parts of the transmission that are not in use. The inventive on/off-function is easy to adapt, because it mainly uses existing components, and no extra valve or control unit has to be introduced in the transmission. The inventive valve member could thereby even be introduced in an existing transmission.
The valve member of the power take off unit can be arranged on any part that is moving with said engagement mechanism. The engagement mechanism preferably comprises:                an engaging sleeve, which when the engagement mechanism is positioned in its first position, engaging a drive shaft of the main gearbox and a driven shaft of the power take off unit, and when the engagement mechanism is positioned in its second position, disengaging said driven shaft of the power take off unit from said drive shaft of the main gearbox,        a shift fork, provided to execute a movement of the engaging sleeve between said first and second position, and        a piston, provided to transmit a shift force from a drive source to the shift fork to execute said movement.        
Said valve member is arranged on one of said engaging sleeve, shift fork or piston. Obviously, each of these parts can be divided into two or more subparts and/or having other parts moving along with them. The valve member can be arranged, on either of said parts or any other part moving with the engaging mechanism. It is up to designer of the transmission to choose on which part the valve member should be located; e.g. dependent of the available space in the housing of the power take off unit and the location of the lubricant inlet. The choice of location of the valve member on either of these parts does not affect the object of the invention. In fact, the designer's freedom of choice of location of the valve member indicates the versatility and the strength of the invention.
In an embodiment of the inventive valve member, it comprises a rod protruding from the engaging mechanism, i.e. one of said parts of the engagement mechanism. When the engaging mechanism is moved from its first to its second position, said rod is guided into the lubricant inlet, which thereby is blocked. The rod and the lubricant inlet are thereby adapted such that, when the rod is positioned inside the inlet, no lubricant can pass through the lubricant inlet.
In another embodiment of the inventive valve member, it comprises a surface located on the engagement mechanism, i.e. on one of said parts of the engagement mechanism. Said surface is adapted such that, when the engaging mechanism is moved from its first to its second position, said surface is slid over the oil inlet, such that the oil inlet is blocked. Further, said surface of the valve member as well as the surface of the lubricant inlet are thereby adapted such, that they have a tight fit, when said surface of the valve member is located in front of the lubricant inlet.
Both of the two embodiments of the valve member described above, can be located either on the engagement sleeve, the shift fork, the piston or any other part moving along with the engagement mechanism. For the invention, the specific design of the valve member is not important. It can be adapted to be formed in one piece with said parts or be mounted thereon, as long as the valve member fulfils its purpose; i.e. to prevent lubricant to flow through the lubrication inlet when the power take off unit is disengaged.
It is not given from the invention that the main gearbox and the power take off unit have a common lubrication system. However, the invention shows superior application in a transmission having a main gearbox and a power take off unit with a common lubrication system.
The lubricant in the lubrication circulation system can be a mineral or synthetic oil or any other suitable lubricant.
Preferably, the lubricant is circulated in the lubrication system by a pump, which can be either mechanically or electrically driven. A forced circulation of the lubrication is beneficial to secure a needed cooling and lubrication of all the parts in the power take off unit as well as the main gearbox. The choice of powering method—mechanically or electrically—is left to the designer and is not of importance for the invention. The valve member is designed to have two positions, to allow a flow of lubricant and to prevent a flow of lubricant. The flow rate of the lubricant is dependent the pump of the lubrication system. Usually, the pump is driven by the motor/engine to which the transmission is connected. Thereby is also the flow rate of the lubricant dependent on the output power of the engine, hence a greater power out put results in greater flow rate of the lubricant.
Commonly, a pneumatic, electrical or hydraulic power source drives the engaging mechanism. Either of these control methods are suitable for the invention.
The inventive transmission is adapted to be mounted in a vehicle; preferably a utility vehicle such as a tractor, a dump truck or a crane truck. However, the inventive transmission can also be adapted for industrial applications.